1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device of an inversion method capable of lowering power consumption and preventing deteriorated picture quality.
2. Description of the Conventional Art
LCD devices are widely used as the next generation display devices, replacing conventional cathode ray tubes (CRT) because of the advantages of LCD devices, such as a high picture quality, a low power consumption, a light weight, and the like.
LCD devices use the optical anisotropy of liquid crystals to display an image by controlling the transmittance of light supplied from a light source. The transmittance of the light is controlled by applying an electric field to liquid crystals contained between a thin film transistor array substrate and a color filter substrate, thereby rearranging the liquid crystals.
Generally, LCD devices are manufactured using twisted nematic (TN) liquid crystals. The TN liquid crystal is driven by a vertical electric field of a common electrode formed on the thin film transistor array substrate and a common electrode formed on the color filter substrate. However, the light transmittance of the TN liquid crystal changes according to the viewing angle in right and left directions which limites the fabrication of large area LCD devices.
That is, in a TN LCD device driven by a vertical electric field, the light transmittance is symmetrically distributed according to a viewing angle in right and left directions but is asymmetrically distributed according to a viewing angle in up and down directions. Accordingly, image inversion is generated in up and down directions thereby narrowing the viewing angle.
In order to solve this problem, an in-plane switching (IPS) method for driving the liquid crystal using a horizontal electric field has been proposed.
The IPS LCD device enhances viewing angle characteristics such as contrast, gray inversion, and color shift, as compared to an LCD device where the liquid crystal is driven using a vertical electric field. Therefore, the IPS LCD device obtains a wider viewing angle. Accordingly, the IPS method is widely used in LCD devices with a large display area.
FIG. 1 is an exemplary view illustrating a planar construction of a thin film transistor array substrate in a general IPS LCD device. As illustrated in FIG. 1, the LCD device comprises: a plurality of gate lines GL1˜GLn arranged on a substrate in a horizontal direction; a plurality of common voltage lines CL1˜CLn arranged to be alternate with the gate lines GL1˜GLn on the substrate in a horizontal direction; a plurality of data lines DL1˜DLm arranged on the substrate in a vertical direction, perpendicular to the gate lines GL1˜GLn; and a plurality of pixels P1 formed at each intersection between the gate lines GL1˜GLn and the data lines DL1˜DLm. Each pixel P1 is provided with a pixel electrode 11 and a thin film transistor T1. The thin film transistor is generally used as the switching device.
As illustrated, the source electrode of the thin film transistor T1 is connected to the data lines DL1˜DLm, the gate electrode is connected to the gate lines GL1˜GLn, and the drain electrode is connected to the pixel electrode 11.
The pixel P1 is provided with not only the pixel electrode 11 but also a common electrode 13. The common electrode 13 is electrically connected to the common voltage lines CL1˜CLn, and is arranged in the pixel P1 to be alternate and in parallel with the pixel electrode 11.
In the IPS LCD device, when a scan signal is sequentially applied to the gate lines GL1˜GLn from a gate driving unit (not shown), the thin film transistors T1 of which gate electrodes are connected to corresponding gate lines GL1˜GLn are turned on by the potential of the scan signal. Also, image data outputted from a data driving unit (not shown) is applied to the pixel electrode 11 through the source electrode of the thin film transistor T1.
A common voltage is applied to the common electrode 13 through the common voltage lines CL1˜CLn, so that a voltage difference is generated between the pixel electrode 11 and the common electrode 13 arranged in parallel with each other. The voltage difference generates a horizontal electric field thereby re-arranging the liquid crystal inside the pixel P1. The arrangement of the liquid crystal changes according to the size of the electric field thereby varying the transmittance of the light supplied from a lamp. Since the common voltage lines CL1˜CLn are electrically connected to each another, the same voltage is applied to each common electrode 13 through the common voltage lines CL1˜CLn.
As aforementioned, since the scan signal outputted from the gate driving unit is sequentially applied to each gate line GL1˜GLn for one frame, the pixels P1 corresponding to each gate line GL1˜GLn to which the scan signal is not applied have to maintain the arrangement of liquid crystal for one frame thereby to maintain a certain brightness. The common electrode 13 and the pixel electrode 11 are separated from each other with liquid crystal there between and serve as a capacitor. Hereinafter, the common electrode 13 and the pixel electrode 11 will be called as a liquid crystal capacitor. Since a charge is filled between the common electrode 13 and the pixel electrode 11 as much as a voltage difference between a common voltage and a voltage according to the image data, the arrangement of the liquid crystal is maintained for one frame. Also, the common electrode 13 formed at the pixel P1 is overlapped with the previous gate lines GL1˜GLn at a certain region thereby to serve as a capacitor and is called as a storage capacitor. The storage capacitor complements a charged capacity of the liquid crystal capacitor.
When a certain electric field is constantly applied to a liquid crystal layer of the LCD device, liquid crystal deteriorates and an afterimage is caused by a direct current voltage component. In order to prevent the liquid crystal from deteriorating and to remove the direct current voltage component, a voltage of image data is applied to the liquid crystal layer to repeat a positive voltage and a negative voltage on the basis of the common voltage, which is called as an inversion method.
The inversion driving method includes a frame inversion method that supplies the polarity of the image data to the liquid crystal layer by inverting at each frame; a line inversion method that supplies the polarity of the image data to the liquid crystal layer by inverting at each gate line; and a dot inversion method that supplies the polarity of the image data to the liquid crystal layer by inverting adjacent pixels and at each image frame.
The dot inversion method decreases screen distortion such as a flicker or a cross talk, therefore it the dot inversion method is generally used to fabricate an LCD device.
FIG. 2 is an exemplary view illustrating a voltage waveform of a pixel in according to the dot inversion method. As illustrated in FIG. 2, a common voltage Vcom is sustained as a direct current voltage at a certain level, and scan signals Vgate1˜Vgate3 are sequentially applied to gate lines during each frame.
Image data Vdata is applied to adjacent pixels by inverting the data into a positive voltage and a negative voltage based on the common voltage Vcom. In addition, the image data is applied at consecutive frame units by inverted into a positive voltage and a negative voltage based on the common voltage Vcom.
The image data Vdata applied to the pixel electrode during a turn-on period of the thin film transistor, to which the scan signals Vgate1˜Vgate3 are applied as a high potential, is shown as a waveform of a pixel voltage Vp. The image data Vdata is charged in the pixel, up to a desired level, while the scan signals Vgate1˜Vgate3 of a high potential are applied to the thin film transistor.
When the scan signals Vgate1˜Vgate3 are changed into a low potential, the gate electrode and the drain electrode of the thin film transistor overlap thereby generating a a parasitic capacitance. As a result, the image voltage Vp is lowered, this is referred to as the varied component ΔVp of a pixel voltage. The voltage lowering of the pixel voltage is equally generated at a positive voltage and a negative voltage.
The liquid crystal is driven during a turn-off period of the thin film transistor during which the scan signals Vgate1˜Vgate3 are applied as a low potential by the voltage charged in the pixel.
The voltage obtained by subtracting the common voltage Vcom from the pixel voltage Vp is defined as the liquid crystal driving voltage Vcel (Vdata-Vcom). The arrangement of liquid crystal becomes different according to the size of the liquid crystal driving voltage Vcel. Since the common voltage Vcom is applied to the LCD device as a constant voltage at a certain level, the voltage level of the image data Vdata has to be changed in order to change the liquid crystal driving voltage Vcel. That is, the image data Vdata having a voltage more than the common voltage Vcom has to be applied to the LCD device, and the image data Vdata has to be swung as a positive voltage and a negative voltage based on the common voltage Vcom, thereby increasing consumption power of the LCD device.